The production of bisphenols such as bisphenol-A (BPA) is an important process as bisphenol-A is used in a variety of chemical industries. For example bisphenols are used to produce polymers such as epoxy resins and polycarbonates. In one application, bisphenol-A is reacted with phosgene to produce commercial polycarbonate resins. High quality polycarbonates, such as those used as optical media in the electronics and disk drive industry, requires highly pure bisphenol-A as a reactant. Consequently, much effort has been focused on developing processes to produce bisphenol-A of high purity.
In general, bisphenol-A is produced by a known liquid-phase condensation reaction of phenol with acetone in the presence of an acid catalyst. The reaction product typically includes bisphenol-A, unreacted reactants, by-products of the reaction most notably water, and impurities such as isomers, analogs and homologs. A variety of processes are used to purify and recover bisphenol-A crystals from the reaction product. Purification and recovery of the bisphenol-A typically represents about one half or more of the total capital investment of the system, and known techniques are often very costly.
Phenol, water and acetone are typically removed, often by distillation, prior to recovery of the bisphenol product by crystallization. U.S. Pat. No. 5,783,733 describes one prior art method of producing bisphenol-A wherein phenol and a ketone are reacted in the presence of an ion exchange resin catalyst to produce a reaction product stream including bisphenol-A. Prior to crystallization excess phenol, water and acetone are removed from the product stream. Crystallization, in this case melt crystallization, is used to purify the crude bisphenol, specifically multiple stage fractional melt crystallization with successive steps of crystallization, partial melting (sweating) and total melting is used. Such phenol removal and melt crystallization techniques are very costly in terms of capital equipment and energy consumption.
In another prior art technique, the reaction product stream is fed directly to a crystallizer where a slurry is formed consisting of a liquid phase, and a solid crystal phase of an equal-molar adduct of bisphenol-A and phenol. The adduct crystals are separated from the liquid (referred to as mother liquor) and phenol is removed from the adduct in a series of phenol removal or dephenolation steps. This prior art method is described for example in U.S. Statutory Invention Registration US H1943. The steps to remove phenol from the adduct, such as distillation or nitrogen desorption, are quite costly and add to the complexity of the system. Finally, multiple stage fractional melt crystallization is preformed to produce the product bisphenol-A.
Many variations to the bisphenol-A process have been proposed by the prior art. Another example is described in U.S. Pat. No. 5,723,688, which provides a method of preparing an adduct of bisphenol with a phenolic compound wherein distillation of the reaction product stream prior to crystallization is omitted. Specifically, U.S. Pat. No. 5,723,688 describes a method of reacting a carbonyl compound with a stoichiometric excess of a phenolic compound in the presence of an acidic cation exchange resin to produce a product mixture containing bisphenol, unreacted phenolic compound, unreacted carbonyl compound and water. The product mixture is then crystalized to form an adduct of bisphenol with the phenolic compound. Once formed the adduct is preferably washed with a phenolic compound which undergoes costly treatment such that at least a portion of the phenolic compound has been purified by means of an acidic cation exchange resin and a basic cation exchange resin. To recover bisphenol-A from the adduct, dephenolation steps are employed where the solid adduct is melted and the phenolic compound is recovered by distillation.
While advances have been made in the production of bisphenol-A, further improvements are needed. The aforementioned prior art methods require costly phenol removal steps and/or phenol purification steps, and costly multiple steps to purify the adduct and/or bisphenol. Further, as the purity requirements for bisphenol-A crystals become more rigorous, the complexity and costs of producing bisphenol-A increase. Accordingly, improved methods for producing bisphenol-A are needed.